1. Field of the Invention
The present invention relates to an electromagnetic wave energy seal arrangement for a microwave oven and particularly, to a choke seal member for a door of a microwave oven in which the leakage of electromagnetic wave energy through a gap between the door and a front panel of a heating cavity of the microwave oven can be effectively prevented from the heating cavity and also the thickness of the door of the microwave oven can be reduced.
2. Description of the Prior Art
Various types of microwave ovens are well known in the art as a useful kitchen utensil which cooks foods by producing heat from food itself by radiating high frequency electromagnetic wave generated from a magnetron to foods within a hating cavity. However, when the electromagnetic wave generated from the magnetron in such microwave ovens is radiated to the human body over a predetermined quantity, fatal harm may be inflicted to the human body. Therefore, the leakage of the electromagnetic wave to exterior from the heating cavity of such microwave ovens should be prevented.
It is inevitable that such microwave ovens used in general are made to have a predetermined gap between the door and the front panel of a heating sanity in their structure. Therefore, in order to prevent the leakage of electromagnetic wave energy through the gap, generally a capacitive seal, a .lambda.g/4 choke seal (.lambda.g is the wavelength of electromagnetic wave within the heating cavity), and a ferrite rubber which is a high frequency absorption material, are widely known and having been used.
Such a conventional electromagnetic wave energy leakage preventing device is described in FIG. 2. As shown in FIG. 1, the microwave oven includes a main body 1, a door 2 for opening and closing a heating cavity 8, a glass 3 for viewing through the interior attached to the door 2 so as to be able to see the interior of the heating cavity 8, which is mounted with keeping a predetermined distance with an external glass 3' (FIG. 2). FIG. 2 shows the structure of a choke seal for preventing electromagnetic wave energy leakage formed along with a circumferential edge of the door 2, wherein as shown in the drawing, the substantially rectangular .lambda.g/4 recessed chamber 6 having a predetermined length L and width W is formed by internal and external plate members 5 and 4. The internal plate member 5 is bent in its bottom end portion toward the recessed chamber 6 about the intermediate portion of the choke channel 6. An opening 6' is formed at its lower side and also, the opening 6' is closed with a choke cover 7 made of plastic material which is attached with a ferrite rubber 7a disposed at inside thereof.
And, a capacitive seal is formed between a front panel 1' of the heating cavity 8 and the internal plate member 5 of the recessed chamber 6. A transmission line of the portion where a capacitive seal is formed has wave number of: ##EQU1## wherein, K.sub.0 =2.pi./.lambda., b represents a distance between the front panel 1' and the internal plate member 5. And, wherein when value b is very little, TE mode and TM mode are cutoff and only the transmission is carried out only in the TEM mode. That is, since the characteristic impedance of a parallel transmission line in case of TEM mode is proportional to ##EQU2## electromagnetic wave energy leaked from the heating cavity 8 can be reflected by making either the value b to be very little or the value .epsilon. to be great, wherein .mu. is permeability and .epsilon. is permittivity.
And, an upper horizontal portion 4a of the external plate member 4 for forming the recessed chamber 6 is a plane that impedance is zero, and if seeing the recessed chamber 6 as a transmission line, as shown in FIG. 3, impedance at a point distanced apart as much as a length l from a load Zl is represented by ##EQU3## wherein zo is the characteristic impedance of transmission line, .beta. is a propagation constant, both of which are values determined by the structure of transmission line.
In the above expression, assuming that zl=o, then it becomes Zin=jzo tan .beta.l, when l becomes .lambda.g/4, Zin becomes infinity; when l=.lambda.g/2, then Zin=o. Therefore, as shown in FIG. 2, in case when the electromagnetic wave energy within the heating cavity 8 is leaked from a point A to a point C, and flowed into the interior of the recessed chamber 6 with a path of B from a point D, the point D is located at a point distanced away as much as .lambda.g/4 from a plane 4a where the impedance is zero, so that the impedance of point D becomes infinity. Thus, the recessed chamber 6 produces resonance reflect the electromagnetic wave energy toward the heating cavity 8 and thereby becoming possible to prevent the leakage toward exterior.
However, according to the choke seal of the conventional structure as aforementioned, when the gap between the internal plate 5 of the recessed chamber 6 and the front panel 1' of the heating cavity 8 becomes great, the electromagnetic wave energy is transmitted not only in the TEM mode but also in the TE mode or TM mode, thus .lambda.g/4 choke seal becomes impossible to exert its own function. Therefore, the leakage of electromagnetic wave energy is outstandingly increased.
Furthermore, since the .lambda.g/4 choke seal and the expensive ferrite rubber are used together, the cost of product is increased and since the length and width of the recessed chamber 6 are relatively long and wide, the thickness of the door 2 becomes thicker, and therefore, there has been a problem that it is difficult to make the product in compact.